NASA's Voyager 2, left, was launched on Aug. 20, 1977 from the Kennedy Space Center in Florida, where it was propelled into space on a Titan/Centaur rocket. Voyager 1 was launched a few weeks later, on Sept. 5, 1977.

Though it launched later, Voyager 1 had a faster, shorter trajectory and has traveled farther than Voyager 2.

Originally designed only to visit Saturn and Jupiter, additional flybys of the two outermost giant planets, Uranus and Neptune, were added to Voyager 2's mission after the first two flybys were successfully achieved.

— NASA

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Golden Record

The "Golden Record" is mounted on the Voyager 2 spacecraft at the Kennedy Space Center on Aug. 4, 1977.

Both Voyager spacecraft carry phonograph records that include music and other sounds from Earth and are intended to give any intelligent aliens who might encounter the craft a sense of what life on our planet was like during the latter part of the 20th Century.

— NASA via AP

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Instructions for Extraterrestrials

The "Golden Record" was a 12-inch gold-plated copper disk containing sounds and images.

The cryptic drawings on the front of the record are instructions that explain how the record should be played.

— NASA via AP

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Jupiter's Moon

This color image of the Jovian moon Europa was acquired by NASA's Voyager 2 during its close encounter on July 9, 1979. Europa, the size of our moon, is thought to have a sheet of ice perhaps 100 kilometers thick that overlies a silicate crust.

— NASA/JPL

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Saturn's Rings

Possible variations in chemical composition from one part of Saturn's ring system to another are visible in this archival image from NASA's Voyager 2.

— NASA/JPL

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Saturn's Satellites

This approximate natural-color image from Voyager 2 shows Saturn, its rings, and four of its icy satellites. Three satellites (Tethys, Dione, and Rhea) are visible against the darkness of space, and another smaller satellite (Mimas) is visible against Saturn's cloud tops very near the left horizon and just below the rings. The two dark spots are shadows cast by Mimas and Tethys on Saturn's clouds.

Saturn, the second largest planet in our solar system, is 75,000 miles in diameter at its equator. Because of its rapid spin, however, Saturn is 10% smaller measured through its poles. Saturn's rings are composed mostly of ice particles ranging from microscopic dust to boulders in size.

— NASA/JPL

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Expanded Mission: Uranus

Voyager 2 captured this image of Uranus in 1986.

As the spacecraft flew across the solar system, remote-control reprogramming was used to endow the Voyagers with greater capabilities than they possessed when they left Earth. Their two-planet mission became four.

—

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Expanded Mission: Neptune

Neptune's "Great Dark Spot" and its companion bright smudge are visible in this image captured by Voyager 2.

The Voyager mission was designed to take advantage of a rare geometric arrangement of the outer planets in the late 1970s and the 1980s which allowed for a four-planet tour for a minimum of propellant and trip time.

—

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Triton

Approximately a dozen individual images were combined to produce this view captured by Voyager 2 of the Neptune-facing hemisphere of Triton, the largest of Neptune's 13 moons.

Triton is unusual because it is the only large moon in our solar system that orbits in the opposite direction of its planet's rotation -- a retrograde orbit.

—

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Parting View

The crescents of Neptune and Triton appear in this image acquired by Voyager 2 some 3 days after its closest approach to Neptune in 1989.

After completing their planetary flybys, the spacecraft continued on, flying toward the edge of the heliosphere, the huge bubble of charged particles and magnetic fields that the sun puffs out around itself. Voyager 1 popped free of this bubble in 20013, entering the unexplored realm of interstellar space. Voyager 2 is expected to follow suit in the next few years.